The present invention relates to a multi-layer circuit board with at least two or more layers of circuit patterns connected, and a method of manufacturing same.
With a recent trend of electronic equipment becoming more miniaturized and higher in component density, there is an increasing demand for multi-layered circuit boards in the fields of both industrial and home-use equipment.
In the field of such multi-layer circuit board, there is a strong demand for the development of a method of making inner via-hole connection between multi-layered circuit patterns, and a circuit board having a highly reliable structure. As a method of manufacturing such circuit board, Japanese Laid-open Patent No. 6-268345 is proposing a method of manufacturing a high density circuit board having a novel structure wherein inner via-hole connections are made by conductive paste. This conventional method of manufacturing a circuit board will be described in the following.
A method of manufacturing a circuit board having four layers as a prior art multi-layer circuit board is described in the following.
First, a method of manufacturing a double-side circuit board being the base for a multi-layer circuit board is described.
FIG. 4 is a sectional view of the process for a conventional method of manufacturing double-side circuit boards for inner layers. In FIG. 4, the substrate is formed of prepreg sheet 21. The prepreg sheet 21 has a shape of 250 mm sq. and about 150 xcexcm thick. For example, the prepreg sheet 21 is formed of a composite material having non-woven cloth made from aromatic polyamide fiber and thermosetting epoxy resin impregnated to the non-woven fabric. Releasing films 22a, 22b have a plastic film and a Si type releasing agent applied to one side of the film, and the releasing films 22a, 22b are about 16 xcexcm in thickness. For example, as plastic film, polyethylene terephthalate is employed.
A method of bonding the prepreg 21 and releasing film 22a, 22b together is disclosed in Japanese Laid-open Patent No. 7-106760. Japanese Laid-open Patent No. 7-106760 refers to a method of continuously bonding releasing films 22a, 22b by dissolving the resin component of prepreg 21 with the use of a laminating apparatus.
A through-hole 23 is formed in the pregreg sheet 21 and releasing films 22a, 22b bonded together. The through-hole 23 is filled with conductive paste 24. Metallic foils 25a, 25b such as copper of 18 xcexcm thick are bonded to either side of the prepreg sheet 21. The conductive paste 24 is electrically connected to the metallic foil 25a, 25b. 
In FIG. 4, (a) the releasing films 22a, 22b are bonded to either side of the prepreg sheet 21. Next, (b) the through-holes 23 are formed, at predetermined portions, in the prepreg sheet 21 with the releasing films 22a, 22b bonded thereto, by using a laser beam machining process.
Next, (c) the conductive paste 24 is filled into the through-holes 23. In the method of filling the conductive paste 24, the prepreg sheet 21 having the through-holes 23 is placed on the table of a printer (not shown) and the conductive paste 24 is directly printed on the releasing film 22a. In this case, the releasing films 22a, 22b serve a function as a printing mask and also a function to prevent contamination of the prepreg sheet 21.
Subsequently, (d) the releasing films 22a, 22b are removed from either side of the prepreg sheet 21.
Next, (e) the metallic foils 25a, 25b are laminated to either side of the prepreg sheet 21. And the laminated metallic foils 25a, 25b and prepreg sheet 21 are heated under pressures at a temperature of about 200xc2x0 C. and pressure of about 4 MPa for one hour in a vacuum.
In this way, (f) the prepreg sheet 21 is compressed and becomes reduced in thickness (t2) to approximately 100 xcexcm. At the same time, the prepreg sheet 21 and metallic foils 25a, 25b are bonded to each other. Further, the metallic foil 25a disposed on the surface side and the metallic foil 25b disposed on the back side are electrically connected to each other by the conductive paste 24 filled in the through-holes 23 formed at the predetermined positions.
After that, the metallic foils 25a, 25b are selectively etched, and then circuit patterns 31a, 31b are formed respectively on either side thereof. Thus, a double-side circuit board can be obtained.
FIG. 5 is a sectional view of the process for a conventional method of manufacturing a multi-layer circuit board, and the multi-layer circuit board is a four-layer circuit board.
In FIG. 5(a), a double-side circuit board 40 having circuit patterns 31a, 31b manufactured by the steps from (a) to (g) of FIG. 4 and prepreg sheets 21a, 21b with conductive paste 24 filled in through-holes 23 manufactured by the steps from (a) to (d) of FIG. 4 are prepared.
Next, as shown in FIG. 5(b), metallic foil 25b, prepreg sheet 21b, inner layer double-side circuit board 40, prepreg sheet 21a, and metallic foil 25a are positioned and laminated in this order.
Subsequently, the laminated board formed of these is heated under pressures at a temperature of about 200xc2x0 C. and pressure of about 4 MPa for one hour in a vacuum, thereby curing the prepreg sheets 21a, 21b. Thus, as shown in FIG. 5(c), the prepreg sheets 21a, 21b are compressed and become reduced in thickness (t2) to 100 xcexcm, then the double-side circuit board 40 and metallic foils 25a, 25b are bonded to each other. The circuit pattern 31a and circuit pattern 31b of the double-side circuit board 40 are connected to the metallic foils 25a, 25b by inner via-holes filled with conductive paste 24. Next, as shown in FIG. 5(d), the metallic foils 25a, 25b are selectively etched, thereby forming the circuit patterns 32a, 32b. In this way, a circuit board having four layers can be obtained.
In the above conventional method of manufacturing a multi-layer circuit board, the double-side circuit boards used for inner layers become decreased in strength and rigidity with decrease in thickness of the circuit boards.
That is, as shown in FIG. 6, circuit pattern 31a is formed on the surface of double-side circuit board 40 where circuit pattern 31b is not formed on the back side of same, and the conductive paste 24 of prepreg sheet 21a is compressed and electrically connected to circuit pattern 32 that is the outermost layer. In this case, due to the pressure applied to the conductive paste 24, the double-side circuit board 40 is deformed by the equivalent to the thickness of circuit pattern 31b on the back side of same and comes in contact with the prepreg sheet 21b. 
Accordingly, the conductive paste 24 of conductive paste 24B is less compressed as compared with the conductive paste 24 of conductive paste 24A. As a result, the connection resistance will become unstable.
A multi-layer circuit board of the present invention comprises:
(a) an inner layer circuit board having an inner layer substrate and an inner layer circuit pattern disposed on at least one surface of the inner layer substrate;
(b) a laminated substrate laminated to the surface of the inner layer circuit pattern; and
(c) a laminated circuit pattern disposed on the surface of the laminated substrate,
wherein the laminated substrate includes a plurality of laminated through conductors;
the laminated circuit pattern is electrically connected to the inner layer circuit pattern by the plurality of laminated through conductors;
the inner layer circuit board further includes a smoothing layer;
the inner layer circuit pattern is convex in shape, having a predetermined thickness;
the smoothing layer is disposed on a concave portion of the inner layer substrate where the inner layer circuit pattern is not formed; and
the laminated substrate is laminated to the smoothing layer and the inner circuit pattern.
By the above configuration, the connection resistance between respective circuit patterns can be stabilized.
A method of manufacturing a multi-layer circuit board of the present invention comprises:
(a) a step of preparing an inner layer circuit board, wherein the inner layer circuit board includes an inner circuit pattern disposed on the surface of an inner layer substrate, and the inner layer circuit pattern is convex in shape, having a predetermined thickness;
(b) a step of forming a smoothing layer on a concave portion of the inner layer substrate except the inner layer circuit pattern;
(c) a step of preparing a prepreg sheet;
(d) a step of preparing a metallic foil;
(e) a step of laminating the prepreg sheet to the surface of the smoothing layer and the inner layer circuit pattern disposed on the inner layer circuit board and laminating the metallic foil to the surface of the prepreg sheet, where the prepreg sheet includes a conductive material disposed in a plurality of through-holes;
(f) a step of heating under pressures the laminated inner layer circuit board, the prepreg sheet and the metallic foil, whereby the conductive material forms through conductors; and
(g) a step of forming a laminated circuit pattern by machining the metallic foil.
By the above configuration, deformation of the inner layer circuit board can be prevented when a laminated substrate is laminated to one or either side of the inner layer circuit board by heating under pressures. Further, deformation of the prepreg sheet which forms the laminated substrate can be prevented. Accordingly, a plurality of through conductors for each of the inner layer circuit board and laminated substrate are formed in same length. As a result, the connection resistance between respective circuit patterns becomes stabilized.